1. Field of Invention
The present invention relates to a circuit and a method for controlling a light emitting device, and particularly to such circuit and method which are capable of reducing the pin number of an integrated circuit. The present invention also relates to an integrated circuit for use therein.
2. Description of Related Art
Referring to FIG. 1, conventionally, a light emitting device controller circuit includes an integrated circuit 20, wherein the integrated circuit 20 includes a power stage controller circuit 21 for controlling the operation of a power transistor in a power stage circuit 60 to convert an input voltage Vin to an output voltage Vout for supplying to multiple channels of light emitting diodes (LEDs) CH1-CHn. The power stage circuit 60 for example may be, but is not limited to, synchronous or asynchronous buck converter, boost converter, buck-boost converter, inverter converter, or fly-back converter, as shown in FIGS. 2A-2G. Depending on each application, the power transistor or diode in the power stage circuit 60 may be integrated into the integrated circuit 20 in certain circumstances.
To control the brightness of each LED channel, FIG. 1 shows a current source circuit CS1 formed by an operational amplifier OP1, a transistor Q1, and a resistor R1, for controlling the current through the first LED channel CH1; a current source circuit CS2 formed by an operational amplifier OP2, a transistor Q2, and a resistor R2, for controlling the current through the second LED channel CH2; and so on. Since multiple LEDs are connected in each LED channel, the output voltage Vout is quite high; hence, the transistors Q1-Qn should be devices capable of sustaining high voltage. However, such devices can not be integrated into the integrated circuit 20, but need to be provided outside of the integrated circuit 20. Therefore, to control N LED channels, the integrated circuit 20 requires pins P1-P2N, twice the number of the channels.
In certain cases as shown in FIG. 3, it is required to obtain the drain signals of the transistors Q1-Qn; however, to connect these nodes to the integrated circuit 20 further increases the pin number to three times of the channel number.
In view of the foregoing, the present invention provides a circuit and a method for controlling alight emitting device, with which the pin number of the integrated circuit is reduced.